This invention relates generally to gas turbine engines, and more particularly to shrouds made of a low-ductility material in the turbine sections of such engines.
A typical gas turbine engine includes a turbomachinery core having a high pressure compressor, a combustor, and a high pressure turbine in serial flow relationship. The core is operable in a known manner to generate a primary gas flow. The high pressure turbine (also referred to as a gas generator turbine) includes one or more rotors which extract energy from the primary gas flow. Each rotor comprises an annular array of blades or buckets carried by a rotating disk. The flowpath through the rotor is defined in part by a shroud, which is a stationary structure which circumscribes the tips of the blades or buckets. These components operate in an extremely high temperature environment, and must be cooled by air flow to ensure adequate service life. Typically, the air used for cooling is extracted (bled) from the compressor. Bleed air usage negatively impacts specific fuel consumption (“SFC”) and should generally be minimized.
It has been proposed to replace metallic shroud structures with materials having better high-temperature capabilities, such as ceramic matrix composites (CMCs). These materials have unique mechanical properties that must be considered during design and application of an article such as a shroud segment. For example, CMC materials have relatively low tensile ductility or low strain to failure when compared with metallic materials. Also, CMCs have a coefficient of thermal expansion (“CTE”) in the range of about 1.5-5 microinch/inch/degree F., significantly different from commercial metal alloys used as supports for metallic shrouds. Such metal alloys typically have a CTE in the range of about 7-10 microinch/inch/degree F.
CMC materials are comprised of a laminate of a matrix material and reinforcing fibers and are orthotropic to at least some degree. The matrix, or non-primary fiber direction, herein referred to as interlaminar, is typically weaker (i.e. 1/10 or less) than the fiber direction of a composite material system and can be the limiting design factor.
Shroud structures are subject to interlaminar tensile stress imparted at the junctions between their walls, which must be carried in the weaker matrix material. These interlaminar tensile stresses can be the limiting stress location in the shroud design.
Accordingly, there is a need for a composite shroud structure with reduced interlaminar stresses.